1. Field of the Invention
The present invention relates to a workpiece picking apparatus for picking workpieces sequentially from a group of workpieces using a robot.
2. Description of the Related Art
A robot is used in various industries to hold, transport or otherwise handle a workpiece. These robot applications include what is called “bin picking” in which individual workpieces are picked from workpieces placed randomly in a container or on a pallet. In a bin picking application, the roughly determined position of the workpiece to be picked is calculated based on a two-dimensional image of a wide area taken by an overall search sensor using a CCD camera or the like, and a three-dimensional position and orientation of a specific workpiece is then accurately measured based on the imaging data of a narrow area obtained by an accurate measurement sensor such as a three-dimensional visual sensor. After that, based on the three-dimensional position and orientation of the workpiece thus measured, the workpiece is picked by the robot. Known three-dimensional visual sensors include one type in which a slit light or spot light is projected on an object to form, on the surface of the object, an optical zone or an optical point higher in brightness than the surrounding area, which is observed by an imaging device such as a CCD camera, and then three-dimensional measurement of the object is carried out based on triangulation, and another type in which the position of an object is three-dimensionally measured by a stereo image processing using two imaging devices.
For example, WO97/24206 discloses a composite sensor robot system using a combination of a CCD camera for obtaining the two-dimensional image of a wide area and a laser sensor for measuring the three-dimensional position of a workpiece in a narrow area. Further, Japanese Unexamined Patent Publication No. 2004-50390 discloses a technique for determining picking priority orders for a multiplicity of detected workpieces based on a partial feature or features of each workpiece detected by a camera for imaging a wide area.
As described above, in what is called “bin picking” application, the three-dimensional position and orientation of the workpiece are detected by a combination of the workpiece recognition carried out based on the imaging of a wide area (hereinafter referred to as “overall search”) and the accurate measurement of the three-dimensional position and orientation of the workpiece carried out based on the imaging of a narrow area (hereinafter referred to as “accurate measurement”), and the robot then picks the particular workpiece based on the three-dimensional position and orientation of the workpiece thus detected.
However, in some cases, a workpiece recognized by the overall search cannot be accurately measured. For example, as shown in FIG. 8A, in the case where an overall search sensor (wide area sensor) located above a container box in the figure for imaging a wide area and an accurate measurement sensor (narrow area sensor) located below the overall search sensor in the figure for imaging the specific workpiece in a narrow area are used and the directions to image the workpiece in the overall search and the accurate measurement are different from each other, the workpiece visible by the overall search sensor located at an upper position may be hidden behind another workpiece and not visible by the accurate measurement sensor located at a lower position than the overall search sensor, thereby sometimes resulting in an inaccurate measurement. Also, as shown in FIG. 8B, the workpiece visible by the overall search sensor located at an upper position may be invisible by the accurate measurement sensor located at a lower position due to a wall of the container box or the like.
Further, some workpiece for which the three-dimensional position and orientation can be detected by accurate measurement cannot be held by the robot. For example, in the case where an accurately measured workpiece is located in the neighborhood of a wall of the container box as shown in FIG. 8C, although accurate measurement can be carried out by the accurate measurement sensor, the robot interferes with a wall of the container box and cannot approach the workpiece. Therefore, in the actual workpiece picking apparatus, it is common practice to store in advance the shapes of the container box and robot in a robot control unit, and to check before the robot moves whether the robot will interfere with other objects thereby preventing interference.
In overall search, the workpieces suitable for the accurate measurement and picking are selected by being assigned a priority order using the technique disclosed, for example, in Japanese Unexamined Patent Publication No. 2004-50390 described above. However, the problem described above is difficult to completely avoid. Therefore, unless special consideration is given to the subsequent process for the workpiece which, although detected by overall search, has failed to be successfully picked for some reason such as due to the problem described above, trials for accurate measurement and picking of the particular workpiece are repeated, often resulting in cycle time delay and the suspension of the process.